@MockitoSpyBean requires existing bean instance to spy on

[quaff]

Overview

When an existing bean of the required type is not discovered in the bean factory, Spring Boot's @SpyBean creates a new instance to spy on by instantiating the required type using its default constructor.

Whereas, @MockitoSpyBean requires that a bean of the required type exists and throws an IllegalStateException if an appropriate bean cannot be found.

Consequently, @MockitoSpyBean cannot be used to spy on a bean that does not exist.

Original Description

@MockitoSpyBean cannot spy concrete runtime bean type, but Spring Boot's @SpyBean can.

Example

plugins {
	id 'java'
	id 'org.springframework.boot' version '3.4.0'
	id 'io.spring.dependency-management' version 'latest.release'
}

group = 'com.example'
version = '0.0.1-SNAPSHOT'

repositories {
	mavenCentral()
}

dependencies {
	implementation 'org.springframework.boot:spring-boot-starter'
	testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

tasks.named('test') {
	useJUnitPlatform()
}

package com.example;

import static org.assertj.core.api.Assertions.assertThat;

import org.junit.jupiter.api.Test;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.bean.override.mockito.MockitoSpyBean;
import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;

@ContextConfiguration
@SpringJUnitConfig
public class SpyBeanTests {

	@MockitoSpyBean
	// @SpyBean works
	// @Autowired works
	SubTestService testService;

	@Test
	void test() {
		assertThat(testService.echo("test")).isEqualTo("test");
	}

	@Configuration
	static class Config {

		@Bean
		TestService testService() {
			return new SubTestService();
		}

	}

	static class SubTestService extends TestService {

	}

	static class TestService {
		String echo(String str) {
			return str;
		}
	}
}

Test fails with:

Caused by: java.lang.IllegalStateException: Unable to select a bean to override by wrapping: found 0 bean instances of type com.example.SpyBeanTests$SubTestService (as required by annotated field 'SpyBeanTests.testService')
	at org.springframework.test.context.bean.override.BeanOverrideBeanFactoryPostProcessor.wrapBean(BeanOverrideBeanFactoryPostProcessor.java:242)
	at org.springframework.test.context.bean.override.BeanOverrideBeanFactoryPostProcessor.registerBeanOverride(BeanOverrideBeanFactoryPostProcessor.java:113)
	at org.springframework.test.context.bean.override.BeanOverrideBeanFactoryPostProcessor.postProcessBeanFactory(BeanOverrideBeanFactoryPostProcessor.java:98)
	at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:363)
	at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:197)
	at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:791)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:609)
	at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:221)
	at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:110)
	at org.springframework.test.context.support.AbstractDelegatingSmartContextLoader.loadContext(AbstractDelegatingSmartContextLoader.java:212)
	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:225)
	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:152)
	... 19 more

The workaround is changing the @Bean method to return the most specific type, SubTestService in this example.

[sbrannen]

Sooooo, this turned out to be an interesting one! 😎

Your original example is not doing what you think it's doing.

The following modified version of your example fails with @SpyBean.

@ExtendWith(SpringExtension.class)
class SpyBeanTests {

	// @MockitoSpyBean
	@SpyBean
	SubTestService testService;

	@Test
	void test() {
		MockingDetails mockingDetails = Mockito.mockingDetails(testService);
		MockName mockName = mockingDetails.getMockCreationSettings().getMockName();

		assertSoftly(softly -> {
			softly.assertThat(mockingDetails.isSpy()).as("is spy").isTrue();
			softly.assertThat(mockName).as("mock name").hasToString("testService");
			softly.assertThat(SubTestService.counter).as("instantiation count").isEqualTo(1);
			softly.assertThat(testService.echo("test")).as("message").isEqualTo("@Bean :: test");
		});
	}

	@Configuration
	static class Config {
		@Bean
		TestService testService() {
			return new SubTestService("@Bean");
		}
	}

	static class TestService {
		private final String prefix;

		TestService(String prefix) {
			this.prefix = prefix;
		}

		String echo(String str) {
			return prefix + " :: " + str;
		}
	}

	static class SubTestService extends TestService {
		static int counter;

		SubTestService() {
			this("Default constructor");
		}

		SubTestService(String prefix) {
			super(prefix);
			counter++;
		}
	}
}

If you run it, you'll see that there are multiple failures.

Multiple Failures (3 failures)
-- failure 1 --
[mock name] 
Expecting actual's toString() to return:
  "testService"
but was:
  "example.SpyBeanTests$SubTestService#0"
at SpyBeanTests.lambda$0(SpyBeanTests.java:31)
-- failure 2 --
[instantiation count] 
expected: 1
 but was: 2
at SpyBeanTests.lambda$0(SpyBeanTests.java:32)
-- failure 3 --
[message] 
expected: "@Bean :: test"
 but was: "Default constructor :: test"

What's happening here is that Spring Boot's @SpyBean support does not consider the testService (TestService) bean to be a match for SubTestService, and consequently Spring Boot creates a BeanDefinition for SubTestService which ends up instantiating a new SubTestService using the default constructor.

So, you effectively end up with two beans of type SubTestService, and the Mockito spy that is injected into your test class is not a spy for the SubTestService created via the @Bean method.

In other words, neither @SpyBean nor @MockitoSpyBean can spy on the testService bean of type TestService. But both @SpyBean and @MockitoSpyBean will properly spy on the service returned from the @Bean method if you declare the return type to be SubTestService.

Please note that returning the most specific type from a @Bean method is actually a best practice with Spring.

[sbrannen]

In light of the above findings, I have reworded the title of this issue and added a new Overview section to this issue's description.

[tobias-lippert]

@quaff related issue that I opened in the Spring Boot project and that was closed as it's desired behavior: spring-projects/spring-boot#43245

[quaff]

@sbrannen Thanks for your elaboration, I think @MockitoSpyBean behaves more correctly, we could improve the exception message to remind developers check this:

Please note that returning the most specific type from a @Bean method is actually a best practice with Spring.

EDIT: I created #33965 to improve exception message.